Precious metal recovery using ozone

ABSTRACT

A method for recovering precious metals from ore is disclosed. Ore is treated in an acidic slurry with an activated oxygen mixture obtained from an ultraviolet light ozone generator. The activated oxygen frees chemically bonded precious metals creating an expanded, hydrated ore so that the metals may be oxidized and leached out using standard leaching techniques. In addition, the activated oxygen aids in the leaching process. Substantial increases in the amount of precious metal recovered from a given amount of ore result.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of precious metal recoveryfrom ore. More specifically, it relates to an improved method forrecovering gold and other precious metals contained in ore in an amountnot previously attainable.

2. Description of the Prior Art

Standard recovery techniques used to recover gold and other preciousmetals (such as silver, platinum, iridium, rhodium, palladium, osmium,and ruthenium, hereinafter incorporated in the use of the term "gold")from an ore usually proceed by separating organic material from themetals by thermal or mechanical means, followed by chemical dissolutionof the gold as either the tetracyanide complex, the tetrachloridecomplex, or sometimes as the EDTA complex. Typical examples of orerecovery using cyanide include U.S. Pat. No. 4,188,208 to Guay and U.S.Pat. No. 4,401,468 to Henderson.

In many gold containing ores, minerals such as titanates, aluminasilicates, or iron/aluminum silicates are complexed with the gold. Inthese cases, the availability of gold to the technique of chemicaloxidation followed by dissolution is minimal. (While the Guay patentcited above discusses oxidizing carbonaceous ores with air followed bychlorine, such a process has little effect on these types of ore andgenerally gives poor results.) For example, gold complexed with aluminahas an effective redox potential for oxidation of greater than 1.3volts. This is the effective potential necessary to free the gold fromthe alumina before dissolution by cyanide or chloride is possible. Sincethe effective oxidation potential of free cyanide is only 0.9 volts, andthe oxidation potential of hypochlorite is only 1.1 volts, neither candissolve the gold from the alumina. In the case of titanium silicates,the gold can be even more bound, requiring approximately 1.9 volts. Inother cases, the presence of free iron in the Fe° state, or as ferrousmetal complexes, changes the chemistry from simple oxidation of gold tothe gold/iron couple, thus preventing gold recovery. Basically, gold isoxidized by cyanide, then is reduced by oxidizing the iron, and then isoxidized. Unless the gold is present at greater concentrations than theiron, or some other technology is available, the gold is unrecoverable.Large amounts of gold remain trapped in the ore with a resultant loss,given the high price of gold, of a dramatic amount of economic return.

In addition, intense roasting of many ores can cause the actual loss ofso called micron gold. The boiling point of gold is sufficiently highthat roasting does not actually vaporize the gold; however, very smallparticles of gold can be lost as an aerosol. Unless electrostaticprecipitation apparatus are installed, and particulates are recoveredwith great care, as much as 75% of all gold in a particular type of orecan be lost.

Accordingly, there exists a clear need for a method of recovering alarger proportion of the precious metals contained in ore at an economiccost that has no detrimental side effects or results.

Ozone has long been used in precious metal mining in the destruction ofthe cyanides used for leaching the metals from the ore. Ozone, anallotrope of oxygen, is a pale blue gas, irritating to the nose andmucous membranes, with the formula O₃. It is a very highly reactivemolecule, decomposing upon reaction to normal diatomic oxygen, as wellas to atomic oxygen, a very powerful chemical oxidizing agent. Thehistory of ozone started in the late 1850's with the development of themany electrical experiments done at that time. The uses of ozone havebeen experimentally tested for approximately 100 years, with most of thepotential uses still awaiting discovery. The oxidation potential ofozone is about 2.07 volts depending on the pH and other chemicalproperties of the reaction, or about 47 kcal/mole. Since the bondstrengths of most chemical bonds in molecules are only about 25 to 35kcal/mole, ozone is potentially able to react by bond breaking with mostmolecules. The magnitude of this reactivity is the major benefit ofusing ozone for chemical and biological purposes.

Two methods of commercially producing ozone and other reactive oxygenmolecules currently exist. The first method is the spark sourcegenerator. Such a generator relies on the passage of high energyelectrons from a corona discharge to react with diatomic oxygen in air.Some of the oxygen in the air is converted to atomic oxygen. The oxygenatoms then can react with diatomic oxygen to produce ozone. Despite thelarge capital costs and the sometimes complex technology, mostcommercial ozone production installations have been of the spark sourcetype and this is the type of ozone currently used for cyanidedestruction in precious metal recovery.

In contrast to the energy intensive spark source ozone generators, asecond method of ozone production is possible. It has long been knownthat a layer of ozone resides near the top of the atmosphere. This ozoneis produced by a photochemical reaction between the intense ultravioletradiation of the sun and the oxygen of the atmosphere. Ultraviolet lightcreates ozone by the same mechanism as the spark source generators,namely the reaction of diatomic oxygen with UV light producing atomicoxygen. This atomic oxygen then reacts with diatomic oxygen to produceozone. In addition to the ozone produced, several other reactive speciesare produced in the discharge that are also of use in commercialapplications. This group of compounds produced by a UV ozone generatoris sometimes referred to as activated oxygen. One has to be careful,however, that the design of a UV ozone generator is such that the oxygenis not overly exposed to the UV light because the same UV irradiationthat produces the ozone also destroys it. In addition, the wavelengthused in the generator should be less than 200 nanometers to avoiddestroying the ozone.

At the present time, UV produced ozone is used almost exclusively foraqua filtration and pool and tub disinfection because of its favorablegermicidal effect. Examples of such use are set forth in U.S. Pat. Nos.4,517,084 to Pincon, 4,230,571 to Dadd, 4,189,363 to Beitzel, 4,179,616to Coviello et al., and 3,336,099 to Czulak et al. UV ozone has not beenused in the recovery of precious metals.

SUMMARY OF THE INVENTION

The present invention provides a process for recovering precious metalfrom ore. UV ozone is bubbled through an acidic slurry of ore, breakingdown the chemical complexes that prevent the majority of the preciousmetal from being recovered by standard recovery techniques. After thesechemical bonds have been broken, resulting in a dramatic expansion ofthe ore volume, UV ozone is further used as a substitute for air oroxygen in the typical leaching procedure prior to final recovery of theprecious metal.

Accordingly, it is a primary object of this invention to provide aprocess for allowing the recovery of large amounts of gold and otherprecious metals previously unrecoverable because such precious metal waschemically bound into the surrounding ore's molecular structure.

It is another object of this invention to provide an apparatus forcarrying out the process of this invention. It is yet a further objectof this invention to obtain an expanded precious metal ore capable ofbeing subjected to standard leaching techniques and thereby recoveringlarge amounts of precious metal previously unrecoverable.

Other objects, aspects, and advantages of the invention will be apparentto those skilled in the art upon the reading of the specification andthe appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The use of ozone for gold mining, and in particular, the use ofactivated oxygen for the enhanced recovery of gold, is based on the highreactivity of the activated oxygen species produced in the UV reactor,and on the particular redox chemistry of gold. If one can oxidize thegold from the Au^(o) state to the Au¹ state, then the cyanide orchloride present in the leaching solution can be relied on to recoverthe gold. In many ores the measured redox potential of gold oxidation isgreater than 1.7 volts, however, and only elemental fluorine and ozonepossess the oxidizing power to oxidize gold under these conditions.Fluorine is extremely hazardous and thus is not a viable alternative.

In addition, it has been shown that while spark source ozone generatorswill produce ozone with an oxidation potential of approximately 1.7V to1.9V, UV ozone generators can produce activated oxygen with oxidationpotentials at least as high as 2.4V. In a spark source generator, ozoneis the only oxidizing species present in the discharge, thus precludingreactivity with certain types of molecules requiring higher oxidationpotentials. In the UV reactor-produced ozone, an entire series of"activated oxygen" species are formed that include hydroxyl OH, atomicoxygen, hydrogen peroxide, hydrogen dioxide HO₂, and hydrogen peroxidesdimers and trimers as well as ozone. These additional species are allexcellent oxidizers and thus the reason for the increased reactivity ofactivated oxygen as compared to simple ozone is probably the ability ofOH and 0 radicals to oxidize even very robust chemicals. Conversely, inmost cases the output of a spark source ozone generator will simply nothave sufficient oxidizing power to oxidize the gold. Thus, the use of UVozone is markedly superior.

A key question is why the output of a UV reactor is so different than aspark source generator if the basic reactions to form the ozone are thesame. Complete understanding of the entire process of ozone productionby UV is very sketchy at the present time, since over 17 basic reactionsand 40 secondary reactions occur. A simplified picture, however, is asfollows: Oxygen as it normally occurs in the air is a triplet molecule,meaning that each molecule has two unpaired electrons. UV light, whenreacting with the oxygen, splits the molecule into two radical oxygenswith very high energy content. In contrast, electron bombardment in aspark source reactor produces oxygen radicals with much lower electronicenergies. The high energy atoms produced by the UV reactors then proceedto react with other oxygen and hydrogen-containing molecules in the gasstream. The spark source produced oxygen atoms can react only with otheratoms, (e.g., nitrogen to produce NO_(x)) and with diatomic oxygen. Thesymmetry rule for chemical reactions predicts that photochemicallyproduced oxygen atoms should be at least three times as reactive asatoms produced by either thermal means or electron bombardment. Inpractice, experimentally produced UV oxygen is at least 2.8 times asreactive.

While any type of UV ozone generator will produce the range of speciesdiscussed above and termed activated oxygen, and thus have an increasedreactivity usable with the process of this invention, the proportions ofeach species may vary. U.S. Pat. No. 4,214,962 to Pincon, incorporatedherein specifically by reference, describes a typical UV ozone generator(in that particular case being used for water treatment). Other examplesof UV ozone generators include U.S. Pat. No. 4,329,212 and 4,427,636 toObenshain, and U.S. Pat. No. 4,317,044 to Vaseen. In a preferredembodiment used with the process of this invention, a standard UVgenerator such as that of the Pincon patent is modified so that theairflow is constrained to a much longer path length resulting in agreater residence time for the gasses within the generator. Preferably,a residence time of three minutes is established with a preferredminimum of one and a half minutes. (Care should be taken, however, thatthe residence time is not too long because, as mentioned above, the sameUV that generates the activated oxygen will also ultimately destroy it.)

The longer residence time is the basis for the higher concentrations ofreactive species that are desirable with the process of the presentinvention. A study performed by the University of Georgia using massspectroscopy on the output of a standard UV reactor (as designed by thePincon patent, cited above) gave the following results:

    ______________________________________                                        Species        % of Output                                                    ______________________________________                                        Ozone          69%                                                            Hydroxyl OH    11% (Depending on feed humidity)                               Atomic Oxygen  6.5%                                                           Hydrogen Peroxide                                                                            4.4%                                                           Hydrogen Dioxide HO.sub.2                                                                    2.3%                                                           Hydrogen Peroxides                                                                           6.8%                                                           dimers and trimers                                                            ______________________________________                                    

While such an activated oxygen composition is certainly usable with theprocess of the present invention, it is preferable that the mixturecontain a maximum of 70% ozone, a minimum of 20% hydroxyl, and a minimumof 5% of the remaining compounds listed. In a more preferred embodiment,the mixture contains 40% ozone, 40% hydroxyl, and 5% each of each of theremaining compounds. Such a mixture has a redox potential of 2.4 V. Sucha high redox potential enables the process of the present invention torecover precious metal from almost any type of ore.

In the process of the present invention, the mined ore is put into aslurry in a slurry tank having a mixer and the slurry is acidified withhydrochloric acid, acetic acid, or any of the mineral acids (nitric,sulfuric, hydrochloric, phosphoric, or hydrofluoric) to a pH level of3.0 or below. In a preferred embodiment, acetic acid is added in therange of 1-3% by volume.

UV ozone from a UV ozone generator is then continually sparged into theslurry solution with the object being to maintain the slurry saturatedin activated oxygen. The rate will vary with the amount of ore beingprocessed and with the amount of activated oxygen producible by the UVozone reactor. In a preferred embodiment of the reactor, a large numberof lamps are used for higher irradiation intensity. Since the amount ofactivated oxygen produced depends on the number of photons bombardingthe feed, this allows a greater amount of feed to be processed intoactivated oxygen while avoiding the problems, mentioned above, caused bylengthy residence times.

As discussed above, in many types of ore, precious metal present ispredominantly chemically bound within the ore, being bound to severaldifferent atoms at once and forming a structure somewhat crystalline innature. The oxidation potential required to break out the precious metalfrom this complex is typically too high for any of the normal leachingprocesses, or even spark source ozone, to be effective. As one result ofthis phenomenon, standard fire assays used to determine the gold orprecious metal content of a particular ore may underestimate the actualamount of precious metal present because fire assays will normally onlymeasure the amount of neutral, non-bonded precious metal.

The very high oxidation potential of the activated oxygen mixture beingsparged into the slurry is sufficient to break most of the bonds bindingthe precious metal to the other ore elements. As a direct result of suchbond breaking, the ore being treated will expand, typically up tofifteen to twenty times its original volume. Rather than remaining as acompact, crystalline-like ore, the structure of the ore becomes hydratedand somewhat amorphous. This dramatic increase in volume through thebreaking of chemical bonds allows a much larger percentage of theprecious metal to be reached by the standard type of leaching processes.

After the ore has been expanded, typically for approximately 2 hours,the pH level of the slurry must be raised in order to leach out theprecious metal. Lime (CaCo₃) is added to bring the slurry to pH 7.0 andthen caustic soda (NaOH) is added to raise the pH to 11.0 or higher.

Once the slurry is suitably basic, it is heated to a temperature of150°-200° F., with a preferred embodiment being at 180° F. Ozone hasbeen continuously sparging through the slurry throughout the process tothis point. It is preferable to use a second tank with a heatingmechanism and a sparger for the steps involved once the slurry is to bemade basic. Standard leaching chemicals can now be added using any of anumber of standard procedures known to those of ordinary skill in theart. Such chemicals may include any of the following: cyanide,hypochlorite, ferrous/hypochlorite, thiourea, or thiocarbamate. Theleach chemistry only works in a basic solution on precious metals thathave been oxidized. The present invention uses the activated oxygen tooxidize the precious metal as well after the bonds binding it to theother ore components have been broken because of its outstanding abilityto oxidize. Tests have shown that ozone serves to oxidize gold in acyanide solution at a rate 100 times faster than normal oxygen.

The slurry is allowed to react at the elevated temperature for a periodof time typically ranging from 4 to 6 hours while the activated oxygencontinues to be bubbled through. (Of course, the amount of time allowedcan be more or less than this, but economic maximization generallydictates the prescribed period.) The leach chemicals act to chelate theprecious metal, allowing it to dissolve into the solution.

After reacting the slurry solution, the activated oxygen source isturned off, and the remaining, undissolved ore is allowed to settle. Thechelated precious metal remains in solution. From this point on, thesolution may be pumped off and standard recovery techniques well knownto those skilled in the art are employed to finally recover the preciousmetal in purified form.

Laboratory results and modified pilot plant studies have shown dramaticincreases in gold recovery from gold bearing ores. (See examplesdiscussed below). The economics of ozonation are such that, in manycases, ore that is unprofitable under conventional treatment, and thatwould not normally be worked, can now be worked at a substantial profit.Even tailings from prior ore refining can now be reused to recoversubstantial amounts of precious metal that was unrecoverable using thestandard techniques of the original mining operation. The vast amount oftailings available will now constitute a ready source of preciousmetal..

Ore containing free gold or "leachable" gold can also be profitablytreated with activated oxygen. In the case of free gold, ozone greatlyreduces the amount of leaching required to dissolve the gold, in manycases either reducing the required concentration of cyanide by 50% orincreasing the concentration of gold in the leaching solution by 35 to50%. The advantages of higher concentrations are obvious--less labor toproduce gold metal, less waste treatment problems, and reduced waterconsumption.

The process of the present invention has been found to work on a largevariety of starting ores including granite, pyrite, clays, sulfides, andbentonite. Typically, clay and sulfide ores have proven the mostdifficult from which to recover precious metals, but the process of thisinvention achieves excellent results even with these hard-to-work-withores. Currently, the only ore found not to be treatable by the patentprocess is quartz, because it appears to require an oxidation potentialof 2.6 V.

It has been found that the method of the present invention isparticularly useful in recovering particles of precious metal thatmeasure below 5 microns as compared to the inefficiency of recoveringsuch miniscule particles by traditional means.

The following sample results will further illustrate the process of thepresent invention and the surprising results obtained through its use.These examples are given by way of illustration and are not intended toact as a limitation on the scope of this invention.

Nine different samples of ore, eight obtained from various locations onSalkatniak Island in Alaska and one from Utah, were processed by themethod of this invention in parallel with standard recovery techniquesand a fire assay. Using the preferred UV ozone generator, which gives aspecies mix of 40% ozone, 40% hydroxyl, and 5% each of atomic oxygen,hydrogen peroxide, hydrogen dioxide HO₂, and the higher molecular weightperoxides, the slurried ore was allowed to expand for 2 hours at a pH ofsubstantially 3.0. After raising the pH, standard leaching techniqueswere used using hypochlorite as the leaching chemical. Finally, atomicabsorption spectroscopy was used to determine the precious metal contentof each sample. The results are tabulated in the following table.

    ______________________________________                                        AMOUNT RECOVERED (OZ./TON ORE)                                                       UV OZONE TREATMENT                                                                             FIRE ASSAY                                            SAMPLE # GOLD       SILVER      GOLD                                          ______________________________________                                        1        5.1        13.1        0.29                                          2        2.7        4.9         0.13                                          3        2.1        5.4         0.17                                          4        2.9        5.6         0.14                                          5        0.3        0.9         0.22                                          6        2.9        0.9         0.11                                          7        3.6        5.4         0.16                                          8        4.5        5.2         0.15                                          9        2.2        5.2         0.16                                          AVERAGE:  2.92       5.18       0.17                                          ______________________________________                                    

As can be seen, for these samples, the average increase in gold recoveryusing the process of the present invention was over 1,600% from 0.17oz/ton ore to 2.92 oz/ton ore. While different ore samples may allow agreater or lesser improvement in gold recovery, such results are fairlytypical.

Thus, a method for recovering, in addition to the gold alreadyrecoverable, previously metals previous unrecoverable by standardtechniques is disclosed. The method produces an expanded, hydrated orethat is amenable to recovery by standard recovery techniques. While theexamples are related to the recovery of gold and silver, this inventionis readily applicable to platinum and members of the platinum group.

While the invention has been particularly shown and described withreference to a preferred embodiment, many other uses and modificationsof the method of this invention will be apparent to those skilled in theart upon reading this specification. The invention, therefore, is notintended to be limited other than by the lawful scope of the followingclaims.

What is claimed is:
 1. A slurry for use in the recovery of preciousmetal, the slurry comprising precious metal and activated oxygen, theactivated oxygen comprising at least ozone, hydroxyl, atomic oxygen,hydrogen peroxide, hydrogen dioxide, and hydrogen peroxide dimers andtrimers.
 2. The slurry of claim 1 wherein the activated oxygen comprisesa maximum of 70% ozone, a minimum of 20% hydroxyl, and a minimum of 5%each of atomic oxygen, hydrogen peroxide, hydrogen dioxide, and hydrogenperoxide dimers and trimers.
 3. The slurry of claim 1 wherein theactivated oxygen comprises 40% ozone, 40% hydroxyl, and 5% each ofatomic hydrogen, hydrogen peroxide, hydrogen dioxide, and hydrogenperoxide dimers and trimers.
 4. A slurry for use in the recovery ofprecious metal, the slurry comprising precious metal and activatedoxygen, wherein the activated oxygen is the product of an ultravioletlight ozone generator.
 5. The slurry of claim 1 wherein the slurrycomprises a saturating amount of activated oxygen.
 6. The slurry ofclaim 1 wherein at least part of the precious metal is chemically boundwithin an ore.
 7. The slurry of claim 6 wherein the ore is selected fromthe group consisting of granite, pyrite, clays, sulfides, and bentonite.8. The slurry of claim 6 wherein the ore is tailings.
 9. The slurry ofclaim 1 wherein the precious metal is selected from the group consistingof gold, silver, platinum, iridium, rhodium, palladium, osmium, andruthenium.
 10. The slurry of claim 1 wherein the precious metal is gold.11. A slurry for use in the recovery of precious metal and activatedoxygen, the activated oxygen comprising at least ozone, hydroxyl, atomicoxygen, hydrogen peroxide, hydrogen dioxide, and hydrogen peroxidedimers and trimers.
 12. The slurry of claim 11 wherein the pH is lessthan
 3. 13. The slurry of claim 11 wherein the slurry further comprisesan acid selected from the group consisting of this is a mineral acidacetic acid, and mineral acids.
 14. The slurry of claim 13 wherein themineral acid is selected from the group comprising nitric acid, sulfuricacid, hydrochloric acid, phosphoric acid, and hydrofluoric acid.
 15. Theslurry of claim 11 wherein the slurry further comprises about 1-3% byvolume acetic acid.
 16. The slurry of claim 11 wherein the preciousmetal is derived from an ore, the ore having a volume in the slurry upto 20 times its original volume.
 17. The slurry of claim 16 wherein thevolume of the ore in the slurry is 15 to 20 times its original volume.18. A slurry for use in the recovery of precious metal, the slurry (a)having a neutral pH and (b) comprising precious metal and activatedoxygen, the activated oxygen comprising at least ozone, hydroxyl, atomicoxygen, hydrogen peroxide, hydrogen dioxide, and hdyrogen peroxidedimers and trimers.
 19. The slurry of claim 18 wherein the slurryfurther comprises lime (CaCO₃).
 20. A slurry for use in the recovery ofprecious metal, the slurry (a) having a basic pH and (b) comprisingprecious metal and activated oxygen, the activated oxygen comprising atleast ozone, hydroxyl, atomic oxygen, hydrogen peroxide, hydrogendioxide, and hydrogen peroxide dimers and trimers.
 21. The slurry ofclaim 20 wherein the pH is at least
 11. 22. The slurry of claim 20having a temperature of 150°-200° F.
 23. The slurry of claim 20 furthercomprising a leaching chemical capable of chelating the precious metalso that the precious metal can dissolve into the slurry.
 24. The slurryof claim 23 wherein the leaching chemical is selected from the groupconsisting of cynaide, hypochlorite, ferrous/hypochlorite, thiourea, andthiocarbamate.
 25. The slurry of claim 23 wherein the leaching chemicalis hypochlorite.
 26. The slurry of claim 23 wherein the precious metalis chelated with the leaching chemical.
 27. A slurry for use in therecovery of gold, the slurry comprising gold and activated oxygen, theactivated oxygen (a) comprising at least ozone and hydroxyl and (b)being the product of an ultraviolet light zone generator.
 28. The slurryof claim 27 wherein at least a part of the gold is chemically boundwithin an ore.
 29. The slurry of claim 28 wherein the ore is selectedfrom the group consisting of granite, pyrite, clays, sulfides, andbentonite.
 30. The slurry of claim 28 wherein the ore is tailing.
 31. Aslurry for use in the recovery of precious metal, the slurry (a) havingan acidic pH and (b) comprising precious metal and activated oxygen,wherein the activated oxygen is the product of an ultraviolet light zonegenerator.
 32. A slurry for use in the recovery of precious metal, theslurry (a) having a neutral pH and (b) comprising precious metal andactivated oxygen, wherein the activated oxygen is the product of anultraviolet light zone generator.
 33. A slurry for use in the recoveryof precious metal, the slurry (a) having a basic pH and (b) comprisingprecious metal and activated oxygen, wherein the activated oxygen is theproduct of an ultraviolet light zone generator.